11.9.1 Scope

Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:

(defun binder (x)   ; x is bound in binder.
   (foo 5))         ; foo is some other function.

(defun user ()      ; x is used ``free'' in user.
  (list x))

In a lexically scoped language, the binding of x in binder would never be accessible in user, because user is not textually contained within the function binder. However, in dynamically-scoped Emacs Lisp, user may or may not refer to the binding of x established in binder, depending on the circumstances:

• If we call user directly without calling binder at all, then whatever binding of x is found, it cannot come from binder.
• If we define foo as follows and then call binder, then the binding made in binder will be seen in user:

  (defun foo (lose) (user))

• However, if we define foo as follows and then call binder, then the binding made in binder will not be seen in user:

  (defun foo (x) (user))

  Here, when foo is called by binder, it binds x. (The binding in foo is said to shadow the one made in binder.) Therefore, user will access the x bound by foo instead of the one bound by binder.

Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.

Ce document a été généré par Eric Reinbold le 13 Octobre 2007 en utilisant texi2html 1.78.